High-powered light-emitting diode (LED) chip technology continues to progress, and LED light sources have been quickly and widely used for various lighting purposes. However, in different lighting purposes, the quality requirements for the light source are not the same. Concerning technological developments of the LED, although efforts have been put into improving brightness, luminous efficiency, color rendering property and color temperature consistency, few have noticed the problem of brightness modulation of the LED. Nowadays LED brightness modulation is achieved by pulse width modulation (PWM). Compared to traditional analog current modulation, the pulse width modulation control method has the advantage to overcome variations of the LED color temperature and wavelength caused by variations of the current, and digital control of the PWM is easier.
However, the LED light source generated by pulse width modulation has a flicking problem during the modulation (100 Hz to 1 KHz). When the LED light source is captured by charge-coupled devices (CCDs) or complementary MOSFET (CMOS) image sensors (cross scanning in vertical: 60 Hz, horizontal: 15750 Hz), it is easy to observe that the on-off pattern of the pulse width modulation produces light and dark ripples shown on the display corresponding to the CCD or CMOS image sensor, so the pulse width modulation can not meet the requirements of the scanning frequency. Thus, the image quality on the display is seriously affected.